1. Field of the Invention
The present invention relates to RADAR and communication systems. More specifically, the present invention relates to methods and apparatus for distortion correction within RADAR and communication receivers.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
High dynamic range receiver amplifiers are in demand in modern RADAR and communication systems. However, the non-linearity within a receiver channel causes the channel output signal to be compressed for large input signals. The primary sources of the non-linearity within, for example, a RADAR receiver channel are the amplifiers (gain stages), frequency conversion mixers and, to a lesser extent, components such as switches and variable attenuators. All of these components contribute to the non-linearity of the overall receiver channel. A convenient method of denoting the linearity of these analog devices is the output two-tone third-order intercept point (IP3). The output third-order intercept point is the theoretical output power level (point) at which the intermodulation distortion components are at an equal amplitude to the fundamental carrier tones. From this definition, it can be seen that a higher third-order intercept point results in an analog device with lower distortion levels for a given output power level.
The goal is to achieve a linear output signal from the receiver channel. However, depending upon the amplification provided by the receiver channel, certain problems occur. For example, if two signals of different frequencies (e.g., f.sub.1 and f.sub.2) are received in the channel, cross-modulation occurs resulting in a pair of undesirable side distortion components in the output signal. The least desirable side distortion components, which are generally cubic in nature, are identified by the terms (2f.sub.1 -f.sub.2) and (2f.sub.2 -f.sub.1). Another example is directed to the non-linearity existing in RADAR receiver applications caused by large ground clutter returns. The non-linear induced distortion exhibits a stronger return signal at a specific frequency than a signal produced by, for example, a small target such as an aircraft or a missile. Thus, the non-linearity causes the clutter return spectrum to spread resulting in the masking of the smaller target signal.
An example of non-linear distortion directed to communication systems includes a communication receiver which receives two or more signals simultaneously. Such an example would include a cable television system. In cable television applications, the signals of multiple channels are simultaneously amplified by a single amplifier. The non-linearity of the single amplifier results in intermodulation distortion that generates signals resulting from cross-modulation of two or more channel signals.
The problems associated with mitigation of the non-linear induced distortion and achievement of a linear output signal from the receiver channel have been addressed in the past. Feedback amplifiers employing resistive feedback loops have been utilized for error correction. In this manner, the output signal can be compared to the input signal. Any error resulting from the comparison is reduced at a summing junction. Voltage division sets the gain and enables the output signal and the input signal to be directly compared to cancel the error. The number of feedback amplifiers becomes excessive and, thus, the scheme is expensive.
The requirement for receiver gain stages that exhibit a high third-order intercept point (IP3) and a high level of inherent linearity has been satisfied by employing very high power class A amplifiers. These class A amplifiers are capable of several watts of output power. To make the gain stages more linear, the linear region must be expanded. Although the Class A amplifier serves this purpose, several disadvantages result including excessive cost, high prime power consumption and large size. Because of the high output saturation power capability of the Class A amplifier, a risk of damaging components immediately following the amplifier exists. Delicate devices such as mixers and SAW filters which require high third-order intercept point amplifiers for input drivers are especially vulnerable to damage from overdriving.
The requirement for high third-order intercept point frequency conversion mixers may also be satisfied through conventional technology. Extremely high level mixers may be used in the receiver or, if greater linearity is required, the mixers may be connected in parallel. The high third-order intercept point is effectively doubled every time the number of parallel mixers is doubled. The use of extremely high level mixers requires local oscillator drive levels approaching one watt. Amplifiers that produce a power level approaching one watt are expensive, use large amounts of prime power and create excessive heat that must be dissipated. Further, using high level mixers in parallel compounds these problems.
Feedforward techniques have been used previously to perform error cancellation. In particular, the techniques have been utilized on individual stages. Because of the need for accurate phase and amplitude match, the techniques are difficult to implement at high frequencies. One such method of extending the effective intercept point of an analog circuit without increasing the output saturation power or the D.C. prime power requirement is known as feedforward intermodulation distortion cancellation. This technique is a variation of feedforward error cancellation used for lower frequency (audio) power amplifiers. Further, this technique deals mainly with an increasing of the linearity of a single amplifier stage or a cascade of amplifiers.
In the feedforward technique, an RF input signal is coupled to a high power amplifier which, unfortunately, generates distortion. The original RF input signal is compared with and subtracted from a sample of the distorted power amplifier output signal to yield the distortion signal generated by the power amplifier. The distortion signal is then passed through a low noise amplifier which provides an error distortion signal. The error distortion signal is transmitted to an output terminal where it is subtracted from the distorted power amplifier output signal to yield a low distortion amplified version of the original RF input signal. This feedforward technique causes the signals to be boosted in amplitude and extends the linear region of the amplifier without extending the saturation point. Thus, overdriving delicate components downstream of the amplifier is not a problem.
Unfortunately, other problems arise in connection with the use of this feedforward technique. For example, the technique is primarily applicable when the input and output frequencies are equal. In order to permit this scheme to function properly, the feedforward circuit architecture described above must be incorporated into each of a plurality of cascaded amplifier stages. Feedforward amplifier stages are used at each of the progressively lower intermediate frequencies (IF). Mixers are employed to assist in the frequency conversion. All frequency conversions are completed prior to the input of the signal into a digital processor. Because the feedforward circuit architecture must be incorporated into each cascaded stage, the circuitry of this technique is space consuming and expensive.
Thus, there is a need in the art for an improvement by which the linear range of a signal receiver may be expanded without increasing the output saturation power, in a cost effective and space efficient manner.